It is well known that color developing agents of the aromatic primary amine type which have been oxidized with exposed silver halides as oxidants react with couplers to produce indophenol, indoaniline, indamine, azomethine, phenoxazine, phenazine and their analogous dyes, thus forming color images.
Of the color images, a magenta color image is formed by using couplers of a 5-pyrazolone, cyanoacetophenone, indazolone, pyrazolobenzimidazole or pyrazolotriazole type.
Most of the magenta color image-forming couplers which have so far been used widely and studies of which have been proceeded are 5-pyrazolones. However, it is known that the dyes formed from 5-pyrazolone couplers have an unnecessary absorption containing a yellow component in the neighborhood of 430 nm to cause color turbidity.
As magenta color image forming nuclei which enable reduction of this yellow component, there have been proposed pyrazolobenzimidazole nuclei in British Patent 1,047,612, indazolone nuclei in U.S. Pat. No. 3,770,447, and pyrazolo[5,1-b]-1,2,4-triazole nuclei in U.S. Pat. No. 3,725,067.
However, the magenta couplers described in the foregoing patent specifications also have such undesirable properties that when mixed with a silver halide emulsion in a condition that they are dispersed in a hydrophilic protective colloid like gelatin, some of them provide only unsatisfactory color images, some of them have low solubility in high boiling organic solvents, some of them are difficult to synthesize, some of them have relatively low coupling activity in an ordinary developer, and that some of them provide dyes of extremely poor fastness to light.
As a result of various searches for new type magenta color image-forming couplers having no side absorption at wavelengths around 430 nm, which is the most serious defect of 5-pyrazolone couplers in respect to hue, some of the present inventors found 1H-pyrazolo[1,5-b]-1,2,4-triazole magenta couples which show no side absorption in the shorter wavelength side, produce dye image of high fastness and can be synthesized with ease as disclosed in JP-A-59-171956 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 4,540,654. These couplers have advantages in that they are excellent in color reproducibility, can be synthesized with ease, and can be readily converted to so-called two-equivalent couplers by introducing a split-off group to a coupling active site, thus achieving reduction of the amount of silver to be used. However, when a coupling split-off group (X) is such a group as to give facility in synthesizing the resulting coupler, e.g., a halogen atom, an alkylthio group, an arylthio group, etc., there arises a problem that such couplers are somewhat inferior to 5-pyrazolone magenta couplers in respect to sensitivity and gradation (gamma). Thereupon, it has been found that couplers having an aryloxy group as a coupling split-off group can afford a means for solving the above-described problem. However, the aryloxy group-releasing couplers suffer from disadvantages that they are obtained in such a low yield as to be unsuitable for large-scaled synthesis and that they have low stability. Also, pyrazoloazoles described in the above-cited U.S. Pat. No. 3,725,067 have similar defects.